Bulging grey heads break the turbid waters of the mighty Mekong river in Cambodia as a pod of rare Irrawaddy dolphins surface to breathe, a thrilling sight, but one that may soon be seen no longer as numbers of the endangered mammal dwindle despite efforts to preserve them. Cambodia has announced tough new restrictions on fishing in the vast river to try to reduce the number of dolphins trapped and killed inadvertently in fishermen's nets. IMAGES AND SOUNDBITES ARRANGED IN SEQUENCES